A plane mirror interferometer can measure the position and/or orientation of objects such as a precision stage in a wafer processing system. For such use, a plane mirror is typically mounted on the stage being measured, and interferometer directs one or more measurement beams for reflections from the plane mirror. Each measurement beam generally corresponds to a separate measurement channel and is combined with a corresponding reference beam for signal processing that produces the measurement. To reduce angular separation between a measurement beam and the corresponding reference beam, some interferometers (commonly referred to as double-pass interferometers) use retroreflectors to direct each measurement beam back for a second reflection from the plane mirror before the interferometer combines the measurement and reference beams. These double-pass interferometers effectively double the path length of the measurement beam, which can have disadvantages.
Interferometer systems that measure the position and orientation of a stage or other object often need to measure multiple degrees of freedom. For example, a rigid three dimensional object generally has six independent degrees of freedom, e.g., X, Y, and Z coordinates indicating a position relative to an X-axis, a Y-axis, and a Z-axis and roll, pitch, and yaw angles corresponding to rotation of the object about the X-axis, the Y-axis, and the Z-axis. In general, at least two of the measurement axes, e.g., the Y-axis and the Z-axis, define directions having at least a component perpendicular to a separation between the interferometer optics and the measurement mirror. Accordingly, an interferometer system measuring all of the degrees of freedom of an object often uses multiple measurement mirrors and interferometer optics in multiple locations around the stage.
Interferometer systems that measure displacement perpendicular to the optics-mirror separation have been developed to prevent interferometer optics from interfering with other processing system components such as a projection lens. U.S. Pat. Nos. 6,020,964 and 6,650,419, for example, describe interferometer systems capable of measuring an altitude of a stage relative to a projection lens. In such systems, a reflector mounted on a stage reflects a measurement beam from a horizontal incident path (e.g., along an X-axis) to a vertical reflected path (e.g., along a Z-axis). A reflector mounted above the stage reflects the vertically directed measurement beam back to the reflector on the stage, where the measurement beam is redirected to a horizontal return path back to the interferometer optics. The total Doppler shift of the measurement beam thus indicates movement along a path having horizontal and vertical components. A separate measurement channel can measure the horizontal component of the motion, so that the vertical component or an altitude measurement can be extracted.
The dynamic range for each degree of freedom measured is generally limited by mirror rotations (e.g., roll, pitch, or yaw rotations), which can deflect the measurement beam, causing the reflected measurement beam to “walk off” the path required for recombination with a reference beam. An acceptable amount of walk off (and correspondingly the dynamic range for a measurement) in general depends on the beam radius w and the optical path length L extending from the interferometer optics to the measurement mirror. For example, the dynamic range for a conventional double-pass interferometer is typically about w/4L radians when measuring a translation along the separation between the interferometer optics and the measurement mirror. The altitude measurements described in U.S. Pat. Nos. 6,020,964 and 6,650,419 are generally subject to similar dynamic range limitations at least because of the need to measure and subtract a horizontal component. To achieve a large dynamic range, conventional interferometers thus require wide beams and/or short separations between the optics and the objects being measured. Large beam widths and short separations are often difficult to accommodate within the space and functional requirements of many systems including wafer processing equipment. Additionally, accommodating large beams increases the size and cost of optical components in the interferometer.
In view of the limitations of existing interferometers, systems and methods are sought that can improve the dynamic measurement range for measurements using plane mirror interferometers without requiring large optical elements or short separations.
Use of the same reference symbols in different figures indicates similar or identical items.